1. Field of the Invention
The present invention is related to polarized light and, more particularly, to measurement of retardance and slow axis azimuth angle, and most especially to systems that produce images of these properties in a two-dimensional image of a sample.
2. Description of the Related Art
Measuring of two-dimensional birefringence distributions is an established technique for analyzing the structure of various specimens. It can also be applied to study the vector or tensor fields associated with birefringence.
The application of two-dimensional birefringence measurements to the analysis of inner stress in construction models using photoelasticity is also well known (Handbook on Experimental Mechanics, Ed. by Albert S. Kobayashi, Prentice Hall: Englewood Cliffs, 1987). E. A. Patterson and co-authors offered a full-field imaging polariscope (E. A. Patterson, W. Ji, and Z. Fwang, “On Image Analysis For Birefringence Measurements in Photoelasticity”, Optic Laser Engineering, 28, pp. 17-36, 1997). It has a circularly polarized illumination beam and six consecutive settings of an analyzer polarizer: left and right circular polarized settings and four linear polarized settings at 0°, 45°, 90° and 135°.
The technique doesn't provide high sensitivity with low retardance specimens, and describes use of a polarization state analyzer comprising a rotated quarter waveplate and rotated linear analyzer.
Imaging polarization techniques have been important for microscope studies of biological specimens (S. Inoué, “A Method For Measuring Small Retardations of Structures in Living Cells”, Exp. Cell Res. 2, pp. 513-517, 1951; S. Inoué and K. R. Spring, Video Microscopy. The Fundamentals, 2nd ed., New York: Plenum Press, 1997; S. Inoué and R. Oldenbourg, Microscopes, in Handbook of Optics, M. Bass, Editor. 1995, McGraw-Hill, Inc.: New York. pp. 17.1-17.52).
Other systems for imaging measurement systems with rotated optical polarization elements have been shown (M. Noguchi, T. Ishikawa, M. Ohno, and S. Tachihara, “Measurement of 2D Birefringence Distribution,” in International Symposium on Optical Fabrication, Testing, and Surface Evaluation, Jumpei Tsujiuchi, ed., Proc. SPIE 1720, 367-378, 1992; Y. Otani, T. Shimada, T. Yoshizawa, “The Local-Sampling Phase Shifting Technique For Precise Two-Dimensional Birefringence Measurement”, Optical Review, 1(1), pp. 103-106, 1994).
J. L. Pezzanitti, and R. A. Chipman proposed a device for measuring Muller matrix coefficients, comprising a polarization state generator and polarization state analyzer. (J. L. Pezzanitti, and R. A. Chipman, “Mueller Matrix Imaging Polarimetry”, Opt. Eng. 34(6), pp. 1558-1568, 1995). The generator and analyzer are created by fixed linear polarizers with parallel transmittance axes and two waveplates, which are rotated with a 5:1 ratio. The waveplate retardances are the same, equal to one-quarter or one-third wavelength. At least 25 consecutive images are required in order to determine a Muller matrix, and in the example given the authors acquire a total of 60 images per measurement.
Y. Zhu and coauthors described two-dimensional techniques for birefringence measurement (Y. Zhu, T. Koyama, T. Takada, and Y. Murooka, “Two-Dimensional Measurement Technique For Birefringence Vector Distributions: Measurement Principle,” Appl. Opt. 38, pp. 2225-2231, 1999). A specimen is illuminated by a beam at three polarization states: one linearly polarized and two elliptically polarized with the same ellipticity value and opposite ellipticity sign, which are obtained by mechanically rotated optical elements. A total of six images are used to obtain the two-dimensional retardance and slow axis azimuth distribution.
A birefringence-mapping device, which contains a mechanically rotated linear polarizer and circular analyzer was described by Glazier and Cosier in 1997 (A. M. Glazer, and J. Cosier, “Method and Apparatus For Indicating Optical Anisotropy,” UK Patent Application No. 2,310,925). Typically, six images of a specimen are taken while the linear polarizer is incremented in 30° steps; these images are then processed to yield the birefringence map, as described in an article (A. M. Glazer, J. G. Lewis, and W. Kaminsky, “An Automatic Optical Imaging System For Birefringent Media,” Proc. R. Soc. Lond. A 452, pp. 2751-2765, 1996). The device is not suitable for measuring low retardance specimens because it is strongly susceptible to light intensity variations, photon statistical noise, detector read-out noise, and digitization error.
Devices with return-path techniques have also been described, by M. I. Shribak “Autocollimating Detectors of Birefringence”, in International Conference on Optical Inspection and Micromeasurements, Christophe Gorecki, Editors, Proc.SPIE 2782, pp. 805-813, 1996; and by M. I. Shribak, Y. Otani and T. Yoshizawa, “Return-Path Polarimeter For Two Dimensional Birefringence Distribution Measurement”, Polarization: Measurement, Analysis, and Remote Sensing II, Dennis H., Goldstein; and David B. Chenault; Eds. Proc., SPIE 3754, pp. 144-149, 1999.
R. Oldenbourg and G. Mei described a method for measurement of retardance and slow-axis azimuth distribution using two techniques: three elliptical and one circular polarized state of illumination beam and circular analyzer; circular polarized state of illumination beam and three consecutive elliptical and one circular polarized setting of analyzer in “Polarized Light Microscopy,” U.S. Pat. No. 5,521,705.
R. Oldenbourg describes a background correction procedure in “Retardance Measurement Method,” U.S. Pat. No. 6,501,548. The method is based on using a universal compensator as an elliptical polarizer/analyzer which is formed by a pair of variable liquid crystal retarders and a linear polarizer.
While there have thus been shown various techniques for retardance measurement and two-dimensional retardance imaging, the existing techniques in the art require taking six or more readings; or are not well-suited to measurement of low-retardance samples; or do not operate with high speed; or offer less than adequate accuracy or noise.